The marginal band (MB) is a continuous peripheral bundle of microtubules, originally discovered in the elliptical, nucleated erythrocytes of non-mammalian vertebrates. Its probable role in generation and/or maintenance of cell ellipticity has not yet been elucidated. In living erythrocytes the MB is oscured by hemoglobin, but partial lysis in modified microtubule polymerization medium containing Triton x-100 renders it visible (and stable) in phase contrast. The semi-lysed cells also contain the nucleus and network of trans-MB material (TBM). Our current working hypothesis for generation of the flattened, elliptical morphology of these erythrocytes involves an interaction between the MB and the TBM. Current and proposed work focuses on the following objectives: (1) development of methods for isolation of free MBs in quantity, (2) analysis of MB proteins by SDS-polyacrylamide gel electrophoresis, (3) study of system lability structurally and biochemically, (4) analysis of free MB ultrastructure by thin sectioning, negative staining, and scanning EM, (5) continued survey of new cell types and species for MB presence and properties, including those of invertebrates. Semi-lysed erythrocytes, representing a simplified, experimentally accessible experimental system, serve as the starting material for most of the proposed work.